Overview

A Checkpoint Table (CKPT) is a hardware array used in a processor register-renaming stage. In the cited RISC-V superscalar processor design, the register-renaming stage includes a Free List (FL), a Register Alias Table (RAT), and the Checkpoint Table.[C1]

Role in register renaming

The CKPT stores snapshots of the RAT. A snapshot is taken whenever an incoming branch instruction is encountered.[C2] Since the RAT holds the most recent mapping from logical register specifiers to physical destination registers, saving RAT snapshots gives the processor a way to recover earlier rename state when speculative execution must be undone.[C3]

Recovery after pipeline flushes

During processor operation, the CKPT buffer helps expedite restoration of the register-renaming state after pipeline flushes. When a mispredicted branch causes a flush, the RAT is restored from the checkpoint that was saved when the offending branch instruction was originally encountered.[C4]

The recovery process also returns to the Free List all physical destination registers allocated after the offending instruction, so those physical registers can be reused.[C5]

Context

The CKPT is described as part of a register-renaming implementation that uses a merged physical register file and a pool of physical registers. In this context, logical register specifiers are replaced with physical register specifiers during renaming, and the CKPT supports speculative execution by preserving recoverable RAT states at branch boundaries.[C6]